[afnog] FreeBSD mirroring

[Phil Regnauld]

On Fri, Apr 28, 2006 at 08:34:07AM +0100, Brian Candler wrote:
> 
> Are there some unwritten assumptions here? e.g.
> 
> (1) that the last block of the original device does not contain any
> important data; and
> (2) that the filesystem will never need to write to the last block of the
> device, after it has been shrunk by one block?
> 
> In other words, how can you be sure that this procedure is safe?

	I don't, I assume gmirror, which has been used in production for
	almost two years now, does the right thing.

	I haven't met any disks yet where the exact disk size, minus
	the label offset, exactly matches a multiple of 512 bytes.

	For example, the slice I create (s1) on my workstation disk:

DISK Geometry:  10011 cyls/255 heads/63 sectors = 160826715 sectors (78528MB)

Offset       Size(ST)        End     Name  PType       Desc  Subtype    Flags

         0         63         62        -     12     unused        0
        63  160826652  160826714    ad4s1      8    freebsd      165
 160826715       9765  160836479        -     12     unused        0          

	Notice the "unused" space at the end, of 9765 sectors.

	If in doubt, create you disks with -1 MB at the end.

	Phil